The major function of the immune system is defense against pathogens. However, even protective responses must be suppressed, either after the invading agent has been successfully neutralized or because the intensity and duration of the response may become dangerous for the host. The mechanisms involved in the regulation of effector responses are quite complex and in many cases still poorly understood. Toxoplasma gondii infection is an excellent model for studying both induction and regulation of Th1 cells since the impairment in either phase of the response is detrimental for the host. For example, mice that lack IL-10 expression succumb during the acute phase of infection with the same kinetics as do IFN-gamma deficient animals, although they display enhanced rather than diminished parasite control. T. gondii-infected mice that cannot signal through IL-27R display a phenotype very similar to IL-10-deficient mice. Recently we have identified an additional and distinct regulatory pathway that relies on induction of glucocorticoids (GC) during T. gondii infection and their effect on T cells. GRfl/fl CD4-Cre mice, which lack GC receptor (GR) expression only in T cells, succumb from T. gondii infection during acute infection. Their decreased survival is not due to increased parasite burden, but instead is associated with significant weight loss, hepatic dysfunction (measured by increased serum levels of aspartate aminotransferase), and muscle cell damage (indicated by increased serum levels of creatine kinase). Importantly, all these disease manifestations and the increased mortality are reversed by the depletion of CD4+ T cells, indicating that toxoplasma-induced pathology in the GRfl/fl CD4-Cre mice may be a consequence of an enhanced Th1 response as previously described in IL-10-/- or IL-27R-/- animals. Detailed comparison of the immune response in T-gondii-infected IL-10, IL-27R or T cell-GR-deficient animals, however, revealed that despite many similarities the underlying mechanisms are different in each case. Thus, in contrast to IL-10 and IL27R deficient animals, infected-GRfl/fl CD4-Cre mice displayed only modest increase in serum levels of IFN-gamma (5-fold). Importantly, this increase in serum IFN-gamma was not associated with systemic increases in IL-12 and IL-27 demonstrating that ablation of GR in T cells did not affect innate immune responses and reinforcing the concept of a pure CD4+ T cell intrinsic defect. Consistent with this hypothesis, the in vivo analyses (both ICS and QT-PCR) showed that the GR-deficient Th1 cells produce more IFN-gamma on a per cell basis during toxoplasma infection. Interestingly, in addition to IFN-gamma mRNA, we detected increased mRNA levels for other cytokines (e.g. IL-2 and IL-10) that indicates that activation of GR in Th1 cells provides a general down-modulatory signal rather than IFN-gamma specific suppressive effect as previously thought. Taken together, our results are consistent with the model proposed for the role of GC during selection in thymus in which production of GC by the thymic epithelial cells decreases the threshold of TCR signaling and, in turn allows selection of T cells with a higher affinity. Based on our study, we propose that during, parasite-induced endogenous GC response plays the same role in the periphery by limiting the strength of CD4+ T lymphocyte activation and that the GC release in the response to stress associated with infection-induced inflammation serves a beneficial but non-redundant role in regulating the extent of the Th1-type response. As noted above mice deficient in IL-10 succumb during the acute phase of infection. The host-protective IL-10 responsible for this effect derives in autocrine fashion from conventional IFN-gamma producing T-bet+ Foxp3neg Th1 cells, which we have shown simultaneously display both effector and regulatory functions. A major project in the lab is to identify the factors regulating IL-10 expression in these CD4 T cells.The similar gene expression profile of IL-10+ and IL-10neg Th1 cells revealed by microarray, and confirmed by QT-PCR, argues strongly these two cell types represent the same Th1 lineage. By, the ChIP-seq analysis of genome-wide H3K4me3 (activating) and H3K27me3 (suppressive) epigenetic modifications, we have demonstrated that IL-10 gene locus is in a different activation state in the two populations: fully active in IL-10+, but only poised in IL-10neg, Th1 cells. In addition to these nonbiased approaches, we have specifically tested the role of specific transcription factors and signaling molecules in the regulation of IL-10 in Th1 cells during T. gondii infection. By employing a model of adoptive transfer of CD4 T cells of interest into RAG-/- animals or when possible, testing intact knock-out mice with either conventional or conditional gene targeting, we have shown that development of IL-10+ Th1 cells during T. gondii infection does not require TCF-1, NFIL3, and STAT-3 but is dependent on signaling through STAT1. Interestingly, the role of STAT1 in the induction of IL-10 in Th1 lymphocytes does not require signaling through the IFN-gamma receptor and is only partially dependent on IL-27 receptor signaling.